Settlement Accumulation Research Articles

Analysing the effect of principal stress rotation on railway track settlement by discrete element method

Principal stress rotation induced by moving loads from trains significantly influences railway track settlement accumulation. The stationary cyclic loading commonly adopted to study railway ballast behaviour under repeated train loading cannot fully represent the effects of principal stress rotation, which needs to be properly considered in both laboratory tests and numerical simulations for a better understanding of ballast deformation behaviour. This paper focuses on studying railway ballast deformation behaviour with an emphasis on particle scale interactions under two different loading scenarios – namely, stationary cyclic and moving wheel loading. A ballasted track model consisting of five sleepers was established based on the discrete-element method (DEM) with realistic polyhedron-shaped elements. The numerical model was validated first based on the testing results from a full-scale high-speed railway testing facility at Zhejiang University. Numerical results clearly indicated that moving wheel loading induced larger principal stress rotation than stationed cyclic loading did. Larger principal stress rotation mobilised higher particle rotation and displacement, which further increased particle rearrangements through individual particle rolling and sliding, and potentially could cause accelerated ballast degradation. It is recommended to consider principal stress rotation in ballast settlement predictions to prevent possible underestimation by stationary cyclic loading and its limitations.

Track settlement restoration of ballastless high-speed railway using polyurethane grouting: Full-scale model testing

Excessive uneven settlements of railway subgrades will reduce the durability of ballastless slab track and jeopardize the train operation safety. In this paper, a technology by injecting polyurethane materials beneath the slab track to uniformly lift the track level is introduced to eliminate track uneven settlement and to ensure its good performance. The mechanical properties and long-term durability of track/roadbed systems that have been rehabilitated by polyurethane grouting are presented. The cyclic uniaxial loading test shows that the polyurethane material has good stability and durability after 3,400,000 cycles of loads; full-scale ballastless track model test shows that the stiffness of track-subgrade system initially decreases by 20% after grouting, resulting in an increase in the track dynamic responses, and after 790,000 cycles of train loads, the stiffness of the grouted subgrade gradually increases while the accumulative settlement is less than 3 mm for maximum 46.12 mm uplift, indicating that the polyurethane-grouted subgrade has good dynamic stability under long-term train loads. Afterwards, this technology was successfully applied in restoring track geometries in an uneven settlement zone of a high-speed railway line on the east coast of China. In-situ tests show that the polyurethane grouting significantly improves the track geometries and control the post settlements after grouting. According to the lab and in-situ tests, it is demonstrated that the polyurethane grouting technology can effectively restore ballastless track uneven settlements and provides adequate long-term durability under moving train loads.

Two decades of settlement of Hong Kong International Airport measured with multi-temporal InSAR

The Hong Kong International Airport (HKIA) was constructed on a platform mainly reclaimed from the sea. The platform has experienced some significant settlement since its operation in 1998 due mainly to consolidation of the materials used in the land reclamation. Although some studies have been carried out to measure the settlement with techniques such as Interferometric Synthetic Aperture Radar (InSAR), the past studies have each only covered some limited time periods. Therefore, a complete history of settlement since the operation of the Airport has never been available to aid the understanding of the spatiotemporal behavior of the land settlement. We attempt for the first time to make full use of most of the available SAR data from multiple SAR sensors (ERS-2, ENVISAT ASAR, COSMO-SkyMed (CSK), and Sentinel-1A) to generate a settlement time series of the HKIA over 1998–2018 with an improved multi-temporal InSAR technique. In order to fill the time gaps between the different SAR datasets, a settlement model is developed based on the InSAR measurements. The results reveal both the spatial and temporal variations of the land settlement. They also show for the first time that the accumulative settlement reaches up to 40 cm over the past two decades. The settlement is largely associated with the material types used in the landfill works and the underlying alluvial sediments as some earlier research has indicated, and the stages of the reclamation works. The results are validated through cross-validation between the datasets and with leveling and GPS measurements on the Airport platform.

Performance of geosynthetic-reinforced pile-supported embankment on soft marine deposit

This study presents field investigations of a geosynthetic-reinforced pile-supported embankment for a transformer substation constructed on a soft marine deposit. The tests were performed following three procedures: construction, surcharge loading and consolidation. The soil stress, the accumulative settlement and the excess pore pressure at different locations in the field were monitored. The results indicated that, with the construction height increasing, the excess pore pressure caused by pile installations in the foundation soil decreased slightly and the pile–subsoil differential settlement increased insignificantly. When the differential settlement was relatively small, the soil arching effect developed rapidly as the differential settlement increased, also showing a partially mobilised characteristic. This partially mobilised arching was weakened under surcharge loading. During the consolidation procedure, the excess pore pressure dissipated remarkably, leading to a noticeable increase in the differential settlement. When sufficient differential settlement was reached, the partially mobilised soil arching developed to the fully mobilised state. At this state, the load-sharing ratio or the pile efficacy (ratio of load borne by the pile cap to the total load) remained steady. Furthermore, for the fully mobilised arching, the pile efficacy was well predicted by four analytical models.

An Experimental Study on Dynamic Model Groove Under Shock Excitation

Introduction: Variation laws of dynamic response and settlement deformation at different depths of soil roadbed were investigated and summarized in this study through a simulation test with a dynamic model groove in soft clay foundation of low road embankment. Materials and Methods: In this test, the traffic load was simulated by single-point and double-point shock excitation. Besides, loading frequency, intensity and duration were adjusted by a single variable control method. Research results show that peak values of soil pressure and pore water pressure decrease with the increase of buried depth, but they are unrelated to loading intensity and frequency. Conclusion: Nevertheless, settlement deformation does not increase linearly with the increase of frequency or time difference. The accumulative settlement at a depth varies as the loading time increases.

Rigid pile improvement under vertical cyclic loading: 1g laboratory small-scale modelling

This research work is a contribution to the understanding of the behaviour of rigid pile reinforcement under a quasi-static vertical cyclic loading. While the system behaviour under static loading is well known, the response of this technique under cyclic and/or dynamic loading is not yet fully understood and cannot be predicted. This work first consists in performing a series of tests on an instrumented small-scale laboratory model, with a scale factor of 1/10 on the lengths. A digital image correlation method is implemented to study the displacement field inside the load transfer platform (LTP). The parametric study performed with the physical model gives an insight on the influence of LTP's thickness and on the presence of a rigid slab at its surface on the behaviour under cyclic loading: a settlement accumulation during the cycles is obtained in all the studied cases; the settlements' accumulation is less for a smaller LTP thickness; and the presence of the rigid slab increases the settlements' accumulation. Nonetheless, its presence results in a reduction of the total settlement during a further monotonic loading. In a second step, the experimental results will serve as a database for the validation of a 3D numerical model.

Nonlinear consolidation of multilayer soil under cyclic loadings

This paper proposes a uniform approach for modelling one-dimensional nonlinear consolidation of multilayer soil under various cyclic loadings based on the differential quadrature method. Various cyclic loading forms and boundary conditions are readily treated. The current differential quadrature formulation using harmonic function coefficients (HDQM) is proven more efficient in solving the consolidation problem of multilayer soil under cyclic loadings than conventional differential quadrature formulation using Lagrangian interpolation polynomial. The computation indicates that the pore pressure and average consolidation degree of a multilayer soil are highly dependent on the loading form and the length of the loading period in each loading–unloading cycle. The influence of the various boundary conditions on the distribution of the pore pressure in soil layers and consolidation rate is also discussed. An analysis on field measurements in Ningbo subway line 1 is presented to demonstrate the application of the proposed method. Analysis reveals that at the initial stage of the subway services, the main settlement is contributed by the consolidation settlement, which is quite considerable, about 0.5 mm per month. When the Up reach 50%, the consolidation settlement tends to stable state, then, the accumulative settlement becomes noticeable.

Soft soil improvement by rigid inclusions under vertical cyclic loading: numerical back analysis

While the behaviour of rigid inclusion improvement systems under monotonic loading is well known, the response under cyclic loading is not yet fully understood and cannot be predicted due to the lack of knowledge about the load transfer mechanisms in the granular platform. This article proposes a three-dimensional numerical modelling under a quasi-static vertical cyclic loading. Numerical results are compared to experimental results obtained on a three-dimensional small scale model, to discuss the numerical procedure and the constitutive models implemented. For the load transfer platform, a shear hardening elastoplastic model is implemented; for the soft soil, the modified Cam Clay model is used, as it features well the compressive and swelling behaviour. Some aspects of the behaviour of the system could be correctly reproduced, such as the loads on inclusions and the settlements under monotonic loading, the settlement accumulation and the decrease of the load transfer onto the inclusions along the cyclic loading, as well as the qualitative influence of a rigid slab placed at the platform surface on the settlement accumulation. However, other features could not be satisfactorily taken into account, namely the behaviour under a rigid slab submitted to cyclic loading in terms of load on the inclusions.

Geodynamics of high-speed railway

Abstract High-speed railway is one kind of important transportation infrastructures developed in many countries worldwide, and it has significant benefits on both economic development and social progress. The most obvious feature distinguishing the high-speed railway from the conventional railway is its high train operation speed, and this leads to strict standards to control railway vibration as well as settlement in design and operations. Geodynamics of high-speed railway becomes an interdisciplinary field of railway engineering, geotechnical engineering, structural dynamics, and has been a hot topic with the dramatic development of high-speed railways world widely. This paper summarizes the recent research advances on geodynamic issues on the high-speed railway, focusing on the theoretical modeling of track-substructure dynamic interaction and consequent wave propagation in soils, distribution of dynamic soil stresses in the soils under track structure and the dynamic amplification as train speed increases, as well as the prediction methods of subgrade accumulative settlement caused by train operations. Finally, some prospective are put forward on the studies on geodynamics of high-speed railways.

MONITORING OF SURFACE SUBSIDENCE OF THE MINING AREA BASED ON SBAS

Abstract. This paper has collected 7 scenes of L band PALSAR sensor radar data of a mine in FengCheng city, jiangxi province, using the Small-baseline Subset (SBAS) method to invert the surface subsidence of the mine. Baselines of interference less than 800m has been chosen to constitute short baseline differential interference atlas, using pixels whose average coherent coefficient was larger than or equal to 0.3 as like high coherent point target, using singular value decomposition (SVD) method to calculate deformation phase sequence based on these high coherent points, and the accumulation of settlements of study area of different period had been obtained, so as to reflect the ground surface settlement evolution of the settlement of the area. The results of the study has showed that: SBAS technology has overcome coherent problem of the traditionality D-InSAR technique, continuous deformation field of surface mining in time dimension of time could been obtained, characteristics of ground surface settlement of mining subsidence in different period has been displayed, so to improve the accuracy and reliability of the monitoring results.

Long-Term Performance of Pile-Supported Ballastless Track-Bed at Various Water Levels

In recent years, the constructions of pile-supported ballastless track-bed have been developed rapidly in China. It appears important to assess the accumulative settlement of this kind of track-bed after years of operation, especially under unfavorable conditions, such as the rising of water level. In this study, a full-scale physical model simulating the pile-supported ballastless track-bed was established. The soil arching effect was previously developed in this model by draining out water in the water bags among the pile caps. The effects of water level and loading cycle on the accumulative settlement of this model were investigated following four testing procedures: water level rising, cyclic loading at high water level, water level lowering, and cyclic loading at low water level. The results indicated that the total accumulative settlement of the track-bed increased rapidly in the beginning of loading and tended to stabilize as the loading cycle increased at high water level, whereas the value varied slightly when loading at low water level. The distribution of the accumulative settlement inside the subgrade at the end of loading with high water level and at the end of this test both presented parabolic shaped variation trends, with the peak point occurring above the water bag and lower values developing above the pile cap. At the high water level, a modified model was applied to estimate the accumulative settlement of the unsaturated zone above the water level (height of soil arch), and the fitting parameters in this model were precalibrated using the testing data. The estimated results revealed that the deformation of the unsaturated zone above the water level accounted for a minor portion of that of the whole tack-bed. By contrast, the zone below the water level is the dominant factor to influence the accumulative settlement of the whole track-bed. From a practical point of view, a well-performing drainage system should be set up to avoid the rising of water level.

Experimental and Numerical Study on the Behavior of Energy Piles Subjected to Thermal Cycles

A laboratory‐scale model test is conducted to improve the understanding of the effects of thermal cycles on the mechanical behavior of energy piles. The model pile is composed of cement mortar and dry sand with a relative density of 30% is used for the model ground. After applying the working load to the pile head, the pile is subjected to three thermal cycles with a magnitude of 15°C. The measured temperature response and mechanical behavior are analyzed and used to validate the proposed numerical approach. In the numerical analysis, the temperature variation due to thermal cycles is calculated using uncoupled heat transfer analysis. Then, the computed temperature field is used as the boundary condition in the sequence stress analysis. A series of numerical sensitivity analyses are carried out using the sequentially coupled method to investigate the long‐term performance of energy piles under different soil and pile head restraint conditions. The numerical results suggest that the restraint condition at the pile head plays an important role in the mechanical response of energy piles. The ultimate pile resistance after thermal cycles does not decrease significantly. The accumulation of settlement of the free head pile and the reduction in the axial force of the restrained head pile should be considered in the design.

Consolidation and Dynamic Response of a Layered Mine Tailings Deposit in Centrifuge Tests

Abstract Tailings from a mining project located in a seismically active region were used to construct and test a layered deposit on the geotechnical centrifuge at the Center for Earthquake Engineering Simulation (CEES) at Rensselaer Polytechnic Institute (RPI). A set of six centrifuge tests was conducted to evaluate consolidation rate, settlement accumulation, shear strength evolution, and response to seismic loading. The material was liquid at disposal, so instrumentation of every layer proved to be a challenge. A total height of about 30 m of mine tailings was deposited in prototype scale (80 g). Brief consolidation of each layer took place at a lower centrifugal acceleration, thus enabling the material to gain some strength and facilitating instrumentation, while simultaneously modeling a few days between disposal of subsequent layers. Miniature pore pressure transducers were used to measure water pressure and settlement, bender elements for shear wave velocity, linear variable differential transformers (LVDTs) for displacement, and miniature accelerometers for dynamic loading. The complete deposit consolidated at 80 g for more than two years in prototype scale, but most settlement, excess pore pressure dissipation, and stiffness increase occurred within the first 0.5 years. A mild slope was excavated at the surface after consolidation and the deposit was then subjected to a moderately strong harmonic motion consisting of 50 cycles. Signs of liquefaction were detected in the pore pressure, acceleration and lateral displacement readings up to mid-depth, and the slope was somewhat flattened after dynamic loading.

Air quality assessment during indoor use of the tobacco heating system THS2.2

The wastes placed at a sanitary landfill contain biodegradable, nondegradable, and semidegradable matters. The biodegradable matters will be rapidly decomposed chemically and biologically into stabilized substances such as humus and inorganic fractions, which is referred to as aged refuse in this book. For the organic wastes, most organic carbon are converted into gases, few into leachate, and others remained as aged refuse. Higher temperature will lead to more rapid degradation and result to higher conversion of the organic carbon to biogas and lower to both solid residues and leachate. The refuse at landfill might be fully mineralized and the accumulative settlement might be over 30% of the initial height after around 22–30 years closure, and postclosure care may be stopped after 22 years, while chemical oxygen demand concentrations in leachate would be reduced naturally to 300 mg/L after over 15 years. A similarity and disparity comparison for different-scale lysimeters with a refuse weight from several hundred kilograms to 10,800 tons, is made, and find that a similarity exists from viewpoint of refuse degradation mechanism and declination tendency of leachate quality and refuse composition among these varied scale lysimeter. An enough weight of waste, such as over 200 kg, should be used at lab when the landfill stabilization process is simulated. The relationships between the refuse components and composition with refuse age are explored in detail and found that the wastes can be considered to be stabilized and become recyclable aged refuse after 8–10 years of placement in China. The air quality at the aged refuse mining point is acceptable and only total bacteria and PM10 may be over the control values. Microbial communities in aged refuse are much more complex than expected. Most of the predominant bacteria in aged refuse exhibit high identity to some extremophiles found in extreme conditions such as highly alkaline and/or saline environments, rocks of an ancient gold mine, old sediments, hypertolerant to arsenic, cold sulfur-rich spring, and the Qinghai-Tibet Plateau permafrost region. The predominated bacilli-like bacteria coexisted with methanogenic archaea, with a rich source of extremophiles, especially alkaliphilic and/or halophilic microbes.

Modelling the settlement behaviour of a strip footing on sloping sandy fill under cyclic loading conditions

This paper investigates the settlement behaviour of a strip footing seating on the crest of an embankment and subjected to cyclic loading. The embankment fill is a dense sand and the issue is the gradual accumulation of settlement over a large number of load repetitions. Cyclic triaxial tests were first conducted to develop a consistent but simple material model for numerical implementation. Particular emphasis was placed on linking the stress-strain behaviour of an unload-reload cycle to the accumulation of permanent strain, with only five input parameters required to model the cyclic behaviour. The material model was implemented in a numerical analysis to compute the settlement behaviour obtained from model tests conducted by another researcher. It is pertinent to highlight that the same soil, compacted to same density at same moisture content, was used for both the cyclic triaxial tests and model tests. Reasonable to good agreement between the experimental and numerical results was achieved.

Evaluation of behaviors of earth and rockfill dams during construction and initial impounding using instrumentation data and numerical modeling

In this study, the behavior of Gavoshan dam was evaluated during construction and the first impounding. A two-dimensional (2D) numerical analysis was conducted based on a finite difference method on the largest cross-section of the dam using the results of instrument measurements and back analysis. These evaluations will be completed in the case that back analysis is carried out in order to control the degree of the accuracy and the level of confidence of the measured behavior since each of the measurements could be controlled by comparing it to the result obtained from the numerical model. Following that, by comparing the results of the numerical analysis with the measured values, it is indicated that there is a proper consistency between these two values. Moreover, it was observed that the dam performance was suitable regarding the induced pore water pressure, the pore water pressure ratio ru, settlement, induced stresses, arching degree, and hydraulic fracturing probability during the construction and initial impounding periods. The results demonstrated that the maximum settlement of the core was 238 cm at the end of construction. In the following 6 years after construction (initial impounding and exploitation period), the accumulative settlement of the dam was 270 cm. It is clear that 88% of the total settlement of the dam took place during dam construction. The reason is that the clay core was smashed in the wet side, i.e. the optimum moisture content. Whereas the average curving ratio was 0.64 during dam construction; at the end of the initial impounding, the maximum amount of curving ratio in the upstream was 0.81, and the minimum (critical) amount in the downstream was 0.52. It was also concluded that this dam is safe in comparison with the behaviors of other similar dams in the world.

Numerical modelling of transient cyclic vertical loading of suction caissons in sand

This paper presents numerical investigations of the monotonic and cyclic behaviours of suction caissons upon vertical transient loading. Both drained and partially drained conditions are investigated. Monotonic compression and traction simulations are carried out to qualitatively compare results with the literature and validate the model. They highlight the different modes of reaction of the caisson to both compression and traction loading. A sensitivity analysis points out the strong influence of some parameters on the resistance of the caisson but also on the failure mechanism. The transient behaviour of the caisson upon different kinds of cyclic load signals is analysed. Results reproduce the settlement and pore water pressure accumulations observed during experiments. The influence of the key design parameters on the settlement accumulation is also assessed. Finally a cyclic diagram is proposed to describe the evolution of the final settlement upon different magnitudes of loading.

Preliminary Testing on High-speed Railway Substructure Due to Water Level Changes

Water infiltration into railway subgrade due to heavy rainfall or flood is one of major factors leading to performance degradation in railways. This paper reported a full-scale physical model test on the performance degradation of a ballastless high-speed railway caused by water level changes. The tests were conducted in a dynamic testing apparatus (5 × 15 × 6 m) developed at Zhejiang University. A portion of a real high-speed railway was constructed in the testing box. Train moving loads were simulated by eight high-performance actuators acting in a predefined sequence. The water levels in the railway substructure were controlled in the testing box by a water storage tank. Both dynamic responses and accumulative settlement of the slab track-subgrade system were monitored. Test results showed that water level rising in subgrade could cause significant performance degradation of the railway. Vibration velocities at the track structure and roadbed increased faster and larger with the train speed after water level rising, as well as the dynamic soil stresses in soils. Accumulative settlement of railway subgrade grew remarkably due to water level rising in subgrade soils. That meant for the high-speed railways in service, reducing water infiltration in railway subgrade was very important to keep railway stable.

Influence of loading path on cyclic mechanical response of small-scale shallow strip footing on loose sand

This paper aims at experimentally exploring the behaviour of a small-scale 1g shallow strip footing on loose dry sand, subjected to vertical and horizontal cyclic loads. An experimental apparatus has been developed and calibrated, and some series of monotonic and cyclic tests with different combinations of load direction and cycle amplitudes have been carried out. Results are presented and discussed in terms of evolution of foundation global stiffness, damping properties, and settlement accumulation along both vertical and horizontal directions with increase in the load cycle’s number. The coupling effect between vertical and horizontal loads is highlighted, underlying its important role even at a low number of cycles, when, in the design practice, the behaviour of the system is often considered to be linear elastic and path independent. The observation also allowed the inference that a variation of the inclination of the cyclic loads may induce significant differences (up to one order of magnitude) in the global stiffness and damping properties. Far from having exhaustively investigated the subject and from the aim of proposing a new empirical quantitative relationship to be directly employed in the design practice, this paper was primarily focused on highlighting some aspects of the mechanical behaviour of the system that are still rather unknown. A further in-depth study allowed the analysis of experimental results within the framework of a meaningful physical interpretation, showing the results to be particularly useful in the perspective of a displacement-based design approach, as required by the current design standards.

Performance of Rocking Systems on Shallow Improved Sand: Shaking Table Testing

Recent studies have highlighted the potential benefits of inelastic foundation response during seismic shaking. According to an emerging seismic design scheme, termed rocking isolation, the foundation is intentionally under–designed to promote rocking and limit the inertia transmitted to the structure. Such reversal of capacity design may improve the seismic performance, drastically increasing the safety margins. However, the benefit comes at the expense of permanent settlement and rotation, which may threaten post-earthquake functionality. Such undesired deformation can be maintained within tolerable limits, provided that the safety factor against vertical loading FSV is adequately large. In such a case, the response is uplifting–dominated and the accumulation of settlement can be limited. However, this is not always feasible as the soil properties may not be ideal. Shallow soil improvement may offer a viable solution and is therefore worth investigating. Its efficiency is related to the nature of rocking, which tends to mobilize a shallow stress bulb. To this end, a series of shaking table tests are conducted, using an idealized slender bridge pier as conceptual prototype. Two systems are studied, both lying on a square foundation of width B. The first corresponds to a lightly-loaded and the second to a heavily-loaded structure. The two systems are first tested on poor and ideal soil conditions to demonstrate the necessity for soil improvement. Then, the efficiency of shallow soil improvement is studied by investigating their performance on soil crusts of depth z/B = 0.5 and 1. It is shown that a z/B = 1 dense sand crust is enough to achieve practically the same performance with the ideal case of dense sand. A shallower z/B = 0.5 improvement layer may also be considered, depending on design requirements. The efficiency of the soil improvement is ameliorated with the increase of rotation amplitude, and with the number of the cycles of the seismic motion.